Electron microscope



Oct. 21, 1941; H-. BOERSCH ET AL ELECTRON MIGROS COPE Filed Nov. 25,1940 Fig.2.

Fig. 5.

Inventors: Hans Boersch Ernst Bnu'che Hans Mahl, ye 52W Then Attorney.

Patented Oct. 21, 1941 ELECTRON Mrcnoscorn Hans Boersch,Berlin-Charlottenburg, and Ernst Briiche and Hans Mahl,Berlin-Reini'ckendorf,

Germany, assignors .to General Electric Com-' ,pany, a corporation ofNew York Application November 25, 1940, Serial No. 367,138 1 In GermanyMarch 17,1939

3 Claims.

The present invention relates to improvements in focused beam dischargedevices'and is' primarily applicable in connection with so calledelectron microscopes. v j

The electron microscope is a device in which magnification effects,analogousto those realized with an optical microscope, are obtained byappropriate refraction of an electron beam which is caused to bereflected from, or projected through, the object to be magnified. Inaccordance with known principles, the desired electron refraction may beobtained bypassing the elec tron beam through an electrostatic ormagnetic field of such character as to produce strong focusing of thebeam.

In connection with magnetic "beamfocusing it has been proposed toaccomplish the desired results by means of a magnetic lens systemcomprising a coil which concentrically surrounds the beam path. Thisarrangement has been found to occasion considerable difficulty,especially in connection with .the positioning of an object to bemagnified at a proper point within the lens system. It is an object ofthe present invention to obviate this difficulty by the provision of asystem in which the magnetic field-producing coil is positionedrelatively remote from the path of the electron beam. In accordance withthe invention, this is accomplished by the use of a,

magnetic structure having opposed tubular magnetic parts whichconcentrically surround the beam and which are connected in anappropriate manner with a coil-supporting core offset from the beampath.

The features which we desire to protect hereinare pointed out withparticularity in the appended claims. The invention itself, togetherWith further objects and advantages thereof may.

best be understood by reference to the following description taken inconnection with the drawing in which Fig. 1 represents a discharge tubeembodying the invention; Fig. 2 is afragmentary view indicating apossible modification of the invention; Fig. 3 isa transversecross-section of a second modification, and Figs. 4 and 5 conjointlyillustrate still a third modification.

Referring particularly to Fig. 1 there is shown a discharge device ofthe cathode ray type which is provided at one end with-means forproducing the impingement of electrons thereof, and thus to provide avisible image Whose nature is determinedby the characterof the impingingelectron stream. I a

In the region between the electrode structure and the viewing window l5there are provided two generally tubular metal elements I! and I8 which,in the case illustrated, form parts of the dischargeenvelope. Inaccordance with the invention, these elements are constituted of aferro-magneticmaterial, for example, iron or ironchrome alloy, and arepositioned in end-to-end relationship so as to be traversed successivelyby the electron beam. 1

At their adjacent extremities the tubular elements l1 and I8 arerespectively providedwith inwardly tapered portions 19 and 20 whichconverge toward central aperturesZl' and 22. It is the function of theelements and of the tapered portions thereof to provide pole'piecesforthe establishment of an axial magnetic field in the gap which separatesthe two elements. Due to the axial symmetry of the elements, the fieldthus provided is itself axiallysymmetrical and is, therefore, welladapted to the-purposesof focusing an electron stream in such fashion asto produce magnification effects.

In order to complete the closure of the discharge device and to renderthe same vacuum tight, the space between the'tubular elements I1 and I8is closed by a metal ring 25 which is of non-magnetic character and ofsuch form as snugly to engage the surfaces of the elements. The metalring 25' is shown as being provided witha removable member,showndiagrammettically at 26, which serves for the introduction into thetube of a mounting fixture for an object which is to be investigated bythe use of the electron microscope. The presence of such a mounting isillustrated diagrammatically at 21.

At a point relatively remote from the beam path and non-concentrictherewith, there is provided a coil 30 having a suificient number ofturns to permit the establishment of a strong magnetic field uponenergization thereof. A low reluctance path for the flux established bythe coil 30 is provided by means of a U-shaped core structure 3| whichis joined at its extremities to the individual elements i1 and, I8, asindicated at 32 and 33respe'ctively. The function of this core, whichshould be of magnetic material, is to permit a strong magnetic field to:be established in the gap between the magneticelements l1 and !8 inspite of the fact thatthe exciting coil itself is appreciably displacedfrom the gap. As a result of this disposition of parts the removal ofthe opening member 26:from:the wall of the discharge vessel for thepurpose of introduction or withdrawal of the object mounting 21 ispossible without the preliminary dismantling of the netic flux such, forinstance, as a permanent magnet.

A possible modification of the invention is lustrated in Fig. 2 whichrepresents the central portion of a cathode ray tube-ethat is, a tubewith the electrode structure and theviewing win: dow and its associatedbulb removed. It is the purpose of this figure to show an arrangement bywhich a series of magnetic lenses having the characteristics of myinvention may be arranged in cascade. This may be accomplished in oneway by the use of a generally cylindrical magnetic. member incombination with two additional elements 35 and 31 of similarcross-section, which are positioned adjacent the respective extremitiesof the element 35. The magnetic lenses which are formed by theseelements, and especially by the inwardly tapered end portions thereof,are energized by a pair of magnetic coils 39 and 45. These in turn areassociated with the base portions of a complex core structure whichcomprises, in efiect, apair of U-shaped cores arranged in side-by-siderelationship. It will be readily understood that if the coils 39 and 40are energized with the proper polarity, the passage of magnetic fluxwill be such as to establish fields in the gaps between the elements 35,3'6, and 31 and thus to create magnetic lens effects in these gaps. Forthe. utilization of these eiiects, an object support may bepositioned inone of the gaps as indicated at 43.

A further aspect of the invention which is also illustrated in Fig. 2comprises the utilization of articulated joints in the magneticstructure, as indicated at and 4.3. By virtue, of the provision of suchjoints it becomes readily possible to separate the parts of thedischarge vessel (i. e. the elements 35, 36 and 31) without completelydismantling the magnetic structure.

A further variation. of the invention is illustrated in Fig. 3 whichrepresents a transverse cross-section taken through a discharge deviceof the general character of that shown in Fig. 1. In this connection itis to be understood that the element represents in section a tubularmagnetic part such as the part I! of- Fig. 1. Connected with this part,and extending radially therefrom in quadrangularly displaced positions,there are provided a number of magnetic core components indicatedrespectively at 5|, 52, '53, and 54. These elements are all associatedwith additional core components to 58 inclusive) which extendperpendicularly thereto. Each of the core structures is assumed to beclosed by the provision of additional leg portions which respectivelyconnect with a second cylindrical element positioned in end-to-endrelationship with the element 50. Moreover, each of the core portions 55to 58 is provided with a coil winding as shown at 60, BI, 62 and 63. Theresultant construction, therefore, represents a multiplication of thearrangement of Fig. 1 in which four U-shaped'core structures, eachexcited by its own exciting coil, are provided at ninety degreepositions around the axis of the discharge device and in which all suchstructures contribute to the energizae tion of a single magnetic lens ofwhich the element 5B constitutes one component.

Figs. 4 and 5 illustrate a still further variation of the invention. Inthis case, tubular pole pieces f extending cylinder 69 of magneticmaterial.

65 and 66 are respectively connected with annular magnetic core. partsBland fi8-which extend outwardly, therefrom. These. corei parts arejoined at their outer peripheries by an axially Y providing a coilwinding either externally or internally of the cylinder 59 (or both), itis possible to establish an axial magnetic field in the gap whichseparates the elements 65 and 66. However, this last describedarrangement is not preferred for the reason that it does not afiordready accessibility to the internal structure of the lens system in themanner which characterizes the embodiments previously described.

While we have described our invention in connection with particularembodiments thereof, it

will baunderstood by thoseslrilled in the ajrtthat;

numerous modifications may be'niade Without. departing from theinvention. We, therefore, aim in the appended claims, to cover all suchequivalent variations as come within the true.

spirit, and scope of the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In combination, a discharge device including means for producing abeam of moving charges, a vacuum-tight discharge enclosure providing anelongated chamber to be traversed bythe beam, said enclosurebeing-iormed in part of a pair of tubular magnetic members positionedv nmu u y c en -end e ation t p vide a lens space between them, magneticfieldproducing means associated with the members for establishingamagnetic, flux through the said lens space to produce a desired focusingefiect on the said beam, and means externally closing the gap betweenthe members to maintain the vacuum-tight quality of the enclosure.

2. In combination, a, discharge device includ: ing means for producing abeam of moving charges, a vacuum-tight discharge enclosure providing arelatively elongated chamber to be traversed by the beam, said enclosurebeing formed in part of a pair of tubular magnetic members positioned inmutually spaced end-to-end relation to provide a lens space betweenthem, a magnetic structure externally connected between the members forestablishing'a mag'netic flux through the said space to produce adesired focusing effect on the said beam, and non-magnetic meansperipherally sealing the gap between the members to maintain the vacuum-tight quality of the enclosure.

3. In combination, a discharge device including means for producing abeam of moving charges, a vacuum-tight discharge enclosure formed inpart of a pair of tubular magnetic members arranged to be successivelytraversed by the beam, said members having conically tapered extremitieswhich are positioned in juxtaposed relation to provide a lens spacebetween them, magnetic field-producing means associated with the'saidmembers for establishing a magnetic flux through the lens space adaptedto produce a desired focusing efiect on the said beam, and a ring ofnonmagnetic material interposed between the'opposed conically taperedfaces of said members for hermetically sealing the gap between them.

' I I-ANS BOERSCH. ERNST BRiicHE. HANS MAHL.

